Perovskite ceramic materials have been used as electronic materials such as dielectric materials constituting multilayer capacitors, piezoelectric materials, and semiconductor materials. Barium titanate has been well-known as typical perovskite ceramic material.
In recent years, demands for miniaturization of electronic components have been increased. Thus, progress has been made in the reduction in thicknesses of sintered dielectric ceramic layers constituting electronic components. To reduce the thicknesses of sintered layers, it is necessary to reduce sizes of crystal grains in the sintered dielectric ceramic layers. In general, high-temperature sintering results in the growth of crystal grains. Thus, material powders such as barium titanate are strongly required to be sintered at low temperatures.
Hitherto, solid-phase methods each including heating a uniform mixture of a titanium oxide powder and a barium carbonate powder to a high temperature of 1,300° C. or higher to undergo a solid-phase reaction have been known as methods for producing barium titanate. However, solid-phase methods have disadvantages that uniform fine particles are not easily obtained and that low-temperature sintering does not easily occur. On the other hand, in wet methods have advantages that uniform fine particles are easily obtained and that the resulting barium titanate powder is easily sintered at a low temperature compared with the solid-phase method. Thus, wet methods are expected to be employed for the production of low-temperature-sintering barium titanate powders. Specific examples of such wet methods include (1) an oxalate method including allowing TiCl4, BaCl2, and oxalic acid to react in an aqueous solution to form the of the precipitate of BaTiO(C2O4)2.4H2O and then thermally decomposing the resulting precipitate; (2) hydrothermal synthesis including subjecting a mixture of barium hydroxide and titanium hydroxide to hydrothermal treatment and then calcining the resulting reaction product; (3) an alkoxide method including hydrolyzing a mixed alkoxide solution containing a barium alkoxide and a titanium alkoxide and then calcining the resulting hydrolysate; and (4) an ordinary-pressure thermal reaction method including calcining a hydrolysate obtained by hydrolysis of a titanium alkoxide in an aqueous barium hydroxide solution.
Although the use of the resulting barium titanate powders obtained by the wet methods slightly reduces sintering temperatures compared with those of powders obtained by the solid-phase methods, the sintering temperatures are high temperatures of 1,200° C. or higher. Disadvantageously, it is difficult to further reduce the sintering temperature.
Thus, various methods for preparing perovskite ceramic materials capable of being sintered at lower temperatures have been reported. Examples thereof include a method of incorporating lithium fluoride into barium titanate (for example, see Patent Document 1); and barium titanate containing, as an accessory component, at least one component selected from alkali metal components, niobium components, alkaline-earth metal components, bismuth components, zinc components, copper components, zirconium components, silica components, boron components, and cobalt components (for example, see Patent Document 2). The development of a material capable of being sintered at a further lower temperature and having a high dielectric constant has been required.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 62-20201
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-173368